It is known when validating coins to perform two or more independent tests on the coin, and to determine that the coin is an authentic coin of a specific type or denomination only if all the test results equal or come close to the results expected for a coin of that type. For example, some known validators have inductive coils which generate electromagnetic fields. By determining the influence of a coin on those fields the circuit is capable of deriving independent measurements which are predominantly determined by the thickness, the diameter and the material content of the coins. A coin is deemed authentic only if all three measurements indicate a coin of the same type.
This is represented graphically in FIG. 1, in which each of the three orthogonal axes P.sub.1, P.sub.2 and P.sub.3 represent the three independent measurements. For a coin of type-A, the measurement P.sub.1 is expected to fall within a range (or window) W.sub.A1, which lies within the upper and lower limits U.sub.A1 and L.sub.A1. Similarly the properties P.sub.2 and P.sub.3 are expected to lie within the ranges W.sub.A2 and W.sub.A3, respectively. If all three measurements lie within the respective windows, the coin is deemed to be an acceptable coin of type A. In these circumstances, the measurements will lie within an acceptance region indicated at R.sub.A in FIG. 1.
In FIG. 1, the acceptance region R.sub.A is three dimensional, but of course it may be two dimensional or may have more than three dimensions depending upon the number of independent measurements made on the coin.
Clearly, a coin validator which is arranged to validate more than one type of coin would have different acceptance regions R.sub.B, R.sub.C, etc., for different coin types B, C, etc.
The techniques used to determine authenticity vary. For example, each coin property measurement can be compared against stored upper and lower limit values defining the acceptance windows. Alternatively, each measurement may be checked to determine whether it is within a predetermined tolerance of a specific value. Alternatively, each measurement may be checked to determine whether it is equal to a specific value, in which case the permitted deviation of the measurement from an expected value is determined by the tolerance of the circuitry. GB-A-1 405 937 discloses circuitry in which the tolerance is determined by the selection of the stages of a digital counter which are decoded when the count representing the measurement is checked.
In a coin validator which is intended for validating a plurality of coin types or denominations each measurement can be checked against the respective range for every coin type before reaching the decision as to whether a tested coin is authentic, and if so the denomination of the coin. Alternatively, one of the tests could be used for pre-classifying the coin so that subsequent test measurements are only checked against the windows for the coin types determined by the pre-classification step. For example, in GB-A-1 405 937, a first test provisionally classifies the coin into one of three types, in dependence upon the count reached by a counter. The counter is then caused to count down at a rate which is determined by the results of the pre-classification test. If the final count is equal to a predetermined number (e.g. zero), the coin is determined to be a valid coin of the type determined in the pre-classification test.
In the prior art, each acceptance window is always predetermined before the test is carried out. Some validators have means for adjusting the acceptance windows. The purpose of the adjustment is to either increase the proportion of valid coins which are determined to be acceptable (by increasing the size of the acceptance window) or to reduce the number of counterfeit coins which are erroneously deemed to be valid (by reducing the size of the acceptance window). Adjustment of the window is carried out either manually, or automatically (e.g. as in EP-A-0155126). In any event, the result of the window adjustment is that the upper and lower limits of the acceptance window are predetermined.
However, by reducing the acceptance windows in order to avoid accepting counterfeit coins, it is possible that genuine coins will then be found to be invalid. Conversely, by increasing the acceptance windows to ensure that a maximum number of genuine coins are found to be valid, more counterfeit coins may also be determined to be valid. The consequence is that adjustment of windows may have adverse effects as well as beneficial effects, and may not increase the "acceptance ratio" (i.e. the ratio of the percentage of valid coins accepted to the percentage of counterfeit coins accepted), or may only increase this ratio by a small amount.
In the field of banknote validation, measurements are also compared with acceptance regions generally of the form shown in FIG. 1. Similar problems thus arise when modifying the acceptance windows to try to avoid accepting counterfeit notes or rejecting genuine notes.